NASA has discovered the smallest planet in a planetary system with a star similar to our sun, paving the way for even more discoveries of sub-Mercury size planets.

NASA has discovered the smallest planet in a planetary system with a star similar to our sun, paving the way for even more discoveries of sub-Mercury size planets.

The Kepler space telescope spotted a new planetary system called Kepler-37, which is 210 light-years away from Earth in the Lyra constellation. The system is made up of three planets, with its smallest, dubbed Kepler-37b, measuring roughly one-third the size of Earth, and only slightly larger than our moon.

Scientists were searching in the “habitable zone” of the planetary system, which is where it is believed liquid water might exist, but data suggests that Kepler-37b does not have an atmosphere, is likely rocky in composition, and cannot support life as we know it.

The other two planets in the system are Kepler-37c, which is slightly smaller than Venus and around three-quarters the size of Earth, and Kepler-37d, which is twice the size of Earth. All three planets orbit their star at less than one-third the distance of Mercury to the sun, making them inhospitable, with temperatures of over 800 degrees Fahrenheit (426 degrees Celsius).

The star itself is of the same class as our sun, but slightly smaller and cooler. Scientists were able to use asteroseismology, a method of analysing sound waves made by the boiling motion under a star's surface, to determine the size of the star, which is pivotal in making out the size of orbiting planets.

“Even Kepler can only detect such a tiny world around the brightest stars it observes," said Jack Lissauer, a planetary scientist at NASA's Ames Research Centre in Moffett Field, California. "The fact we've discovered tiny Kepler-37b suggests such little planets are common, and more planetary wonders await as we continue to gather and analyze additional data.”